With the increasing complexity of telecommunication means, criminals are finding new ways to escape from the surveillance of the police or other law enforcement agencies. This can, potentially, become a threat to society. More specifically this problem is increasing with the global development of mobile telecommunication means.
The term mobile telecommunication can include technologies ranging from cordless telephones, digital cellular mobile radios, and personal communication systems that are evolving to wireless data and networks.
A mobile telecommunication system is usually composed of a base station 13, connected to the public telephone network via a Mobile Telephone Switching Office (MTSO), and of a group of mobile users 14. The base station 13 covers a certain geographical area.
A communication between a mobile user and a standard public network user is thus performed using the base station 13. Each communication link uses a particular frequency band known as a voice channel. The uplink transmission refers to the signal sent by a mobile station 14 to a base station 13, while the downlink transmission refers to a signal sent by the base station 13 to a mobile station 14. Therefore, a single conversation requires two voice channels. The monitoring of a particular conversation implicitly requires the monitoring of the two voice channels. One prior technique for monitoring cellular telephone conversations involves a simple tuner that scans voice channels. This technique is not efficient when the number of communications to handle is high and when the frequency of the voice channel changes often.
A second technique consists of monitoring the two voice channels and the traffic channels of the communication system in order to handle the handover (i.e. when a user, by a physical displacement, changes base stations). Thus it is possible to track a user through a cellular network.
Focusing more precisely on wide band receivers, the prior art comprises one type of system which is composed of a group of front end radio frequency demodulation stages which include parallel narrow-band receivers. Usually, the Radio Frequency (RF) band coverage provided by these narrow-band receivers is of adjacent frequency. Each RF stage has its own local oscillator frequency and Intermediate Frequency (IF) stages. Such a system is very costly because only one RF stage is working at a time. The inactive RF stages are superfluous during operations.
In another configuration (described in patent U.S. Pat. No. 6,002,924 col 1, line 57 to 67), the choice of the first IF and the first local oscillator, the second IF and the second local oscillator allow the user to access a broad band radio receiver. However, because of inter-modulation and image frequency response, certain frequencies are not available. This element is unacceptable for the purpose of this use.
In U.S. Pat. No. 6,002,924, Takano succeeds to create a broadband radio receiver with a continuous spectrum. However, there is still a gap to fill in order to complete the task of intercepting users because for each voice interception, two receivers are needed; this is too costly therefore this is not acceptable. Furthermore, the processing of the RF signal is completed using an analog processing. This analog receiver architecture has the serious drawback to use multiple mixers and filters. Furthermore, the analog receiver architecture is subject to temperature drift and mismatch from part variations.